K2: 8A Control Resistance 2.72 to 3.50 depending on R1

[David Woolley (E.L)]

Hi,

I've  assembled the control board (and most of the front panel.  In
doing the control board resistance checks I've got an apparently out of
range value for the 8A value.  With R1 at the low resistance end of the
track, it is 2k72, which is below the minimum of 3k0, in the manual.
With R1 at the high resistance limit, it is 3k5.  Do I have a problem?
In particular, does the spread given in the manual account for the R1
settings (I would have declared a pass if R1 had been set high enough
initially).

I've checked lots of things without finding a problem.  The resistance
is not obviously polarity sensitive, although it shows a slight positive
dependency on the temperature of the 8V regulator (50R when it was
warmed with an iron set to 75 deg C), so some of it is leakage through
the regulator.

The resistances from the bases of Q1 and Q2 are the approximate 3k9
higher that you would expect if both FET's were cut off.

Do I have anything to worrry about, or should I wait to the first stage
test?
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[Don Wilhelm-3]

David,

You likely have the 4.7k value for RP6 installed which will lower the
resistance measured slightly.  Apparently the 'nominal' resistance value
was not changed in the manual yet to account for the change in RP6 (it
used to be 5.1k).

You do not have a problem.

73,
Don W3FPR

David Woolley wrote:

> Hi,
>
> I've  assembled the control board (and most of the front panel.  In
> doing the control board resistance checks I've got an apparently out of
> range value for the 8A value.  With R1 at the low resistance end of the
> track, it is 2k72, which is below the minimum of 3k0, in the manual.
> With R1 at the high resistance limit, it is 3k5.  Do I have a problem?
> In particular, does the spread given in the manual account for the R1
> settings (I would have declared a pass if R1 had been set high enough
> initially).

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[David Woolley (E.L)]

Don Wilhelm wrote:
>
> You likely have the 4.7k value for RP6 installed which will lower the

It's still 5k1; I actually measured that on some of the ones that are
unterminated, when I was checking for possible shorts.

> resistance measured slightly.  Apparently the 'nominal' resistance value
> was not changed in the manual yet to account for the change in RP6 (it
> used to be 5.1k).
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[David Woolley (E.L)]

David Woolley wrote:
>
> I've  assembled the control board (and most of the front panel.  In
> doing the control board resistance checks I've got an apparently out of
> range value for the 8A value.  With R1 at the low resistance end of the
> track, it is 2k72, which is below the minimum of 3k0, in the manual.
> With R1 at the high resistance limit, it is 3k5.  Do I have a problem?

I *did* have a problem that was indicated by this; even though I thought
I had carefully checked, I mixed up the 5 and 8 volt regulators.  I've
switched them over now, and apart from losing some of the through hole
plating, it seems OK, with a couple of minor reservations.

In the hope that it will help someone searching in the future, the
symptoms, on powering up, were:

- All the segments on the display showed to some extent from all
   angles and dominated the proper display when viewed from above
   the horizontal.

- When the power supply voltage was above, approximately, the 8V
   regulator drop out, it kept sensing slight changes in the pots,
   so the display kept going to various parameter mode change displays.

- The current draw, measured externally, was around 220mA.

- Nothing on the control board seemed to be running warm, but there
   were indications that something on the front panel, near the centre,
   was, although not really hot.

- The internal voltmeter, was showing about 5V when the external supply
   was turned down (I presume this is because it was using the the
   nominal 5V as the reference, which was actually just was close to
   the supply voltage, as the 8V regulator in the 5V supply had dropped
   out.

- No segments lit on the bargraph.

The slight reservations are that the AGC threshold calibration voltage
only reaches 3.79, with R1 on the end stop and the current sense test
showed 0.14 amps (0.08 to 0.22) difference most of the time, with the
odd excursion to 0.16, compared with the manual figures of 0.16 to 0.18.

--
David Woolley
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[David Woolley (E.L)]

David Woolley wrote:

> The slight reservations are that the AGC threshold calibration voltage
> only reaches 3.79, with R1 on the end stop

Oops!  I hadn't set the RF Gain control properly! Now re-calibrated.

--
David Woolley
Emails are not formal business letters, whatever businesses may want.
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that is no longer good advice, as archive address hiding may not work.
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